Twitter tip actions should be tab-selectable, and accessible via enter key

[ryanml]

Currently, you are unable to tab focus twitter tip actions on tweets, you should be able to tab-select them, then hit 'Enter' to engage the tipping banner. The goal is to have this achievable without clicks, for accessibility purposes.

Test plan:

1. Go to your twitter feed or a twitter user's page such as https://twitter.com/BrendanEich
2. Using tab, select through elements until you highlight a tip action
3. Press the enter key
4. Confirm the tip banner appears for the appropriate user
5. Repeat steps 2 - 4 a few times within in the same feed to ensure that the correct user info is shown for different tweets in the banner

[btlechowski]

Verification passed on

Brave	0.70.109 Chromium: 77.0.3865.90 (Official Build) beta (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Ubuntu 18.04 LTS

Verified test plan from the description

Verification passed on

Brave	0.70.112 Chromium: 77.0.3865.90 (Official Build) beta (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan mentioned in the description

Verified passed with

Brave	0.70.112 Chromium: 77.0.3865.90 (Official Build) beta (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.13.6 (Build 17G5019)

• Verified test plan from description
  Able to select Tip button on a tweet with tab:

Able to select with Enter:

Confirmed that selecting Tip button (for a retweet) further down the feed shows the expected info in banner: